Their main concern was that in "the Presidential primary campaign, claims were made that the Earth is not warming, or that warming is due to purely natural causes outside of human control." Or, as some climate-change skeptics would have it regarding excess CO2: They call it pollution, we call it Life.

What was historic was their agreement that absolute certainty is not a prerequisite for environmental policy: "Absolute certainty is unattainable. We are certain beyond a reasonable doubt, however, that the problem of human-caused climate change is real."

In science, certainty has long been a requirement. In brief terms, a scientific theory can be considered proven or disproven if the scientific method has been followed. The theory must be clearly stated, it must be supported by careful experiment or observation that provides precise data enabling a conclusion to be made with certainty. The conclusion will show proof that the theory is correct, or not. Moreover, the process must be capable of independent verification through a repeat of the steps, with results that confirm the conclusion.

This line of thinking has inspired modern society since the Scientific revolution in the 16th century, when the power of empirical and evidence-based data replaced the classical approach in which reason and argument would prevail. Objective data became the ideal, and subjectivity was to be avoided.

So powerful was the scientific approach that it became the foundation of policy and social practice generally, where the elements were quantifiable, technical and later industrial. Married to technology and industry, the whole of society became subject to the scientific paradigm and thus in danger of falling into a global progress trap. Even Stephen Hawking conceded in January 2016 that “Most of the threats we face come from the progress we’ve made in science and technology." **

However in the legal field, the adversarial process remained standard, allowing each side in the argument to present their case. The conclusion would be a matter of judgement, by a jury, a judge, or sometimes a panel of experts. In criminal cases this judgement would be beyond a reasonable doubt.

Eventually it became obvious that the Achilles heel of Science was its insistence on absolute certainty. This can be seen in climate change policies, where skeptics and national leaders have been able to claim, successfully, that anthropogenic climate change had not been conclusively proven, and did not conform to "sound science". Countless products have been released into the global ecosystem unchecked because of the difficulty in proving with certainty that they are harmful. A few that were shown later to be harmful, after incalculable damage, were leaded gasoline, CFCs, PCBs and thalidomide. With the emphasis of 'absolute certainty', science inadvertently provided tools for stagnating true progress, especially in the gravely important areas of global warming and climate instability.

With the acceptance of the principle of "beyond a reasonable doubt" the wise members of the Union of Concerned Scientists and the National Academy of sciences, who wrote and signed the open letter, have provided a historic moment, perhaps even a scientific revolution*. The paradigm shift from obstinate objectivity to reasonable judgement is something to celebrate, even in these dark times.